1. Field of the Invention
The present invention relates to improvements in an X-ray mask and more particularly, to an X-ray mask having an X-ray permeable film made of SiC having a high visible-light transmissivity and also to a method for fabricating the X-ray mask.
2. Description of the Related Art
As higher integration in an integrated circuit is demanded, the importance of micro-processing techniques for circuit patterns, in particular, a lithography technique for forming a pattern on a sensitizer has been recently increased. These days, the lithography technique using visible light as a light exposure medium is used in mass production lines, but its resolution determined by the wavelength of the used visible light is approaching to its limit. Instead, an X-ray lithography technique enabling the remarkable improvement of the resolution in principle has been rapidly developed. In the X-ray lithography, an X-ray exposure mask having a predetermined pattern formed thereon is held parallel to a sample with a spacing therebetween of on the order of 10 .mu.m so that radiation of X-rays from behind the mask causes the mask pattern to be transferred onto the sensitizer on the sample with a unity magnification.
With such a unity magnification transfer system, since the dimensional and positional accuracies of the X-ray mask pattern reflect directly on the device accuracy, the X-ray mask pattern is required to have dimensional and positional accuracies corresponding nearly to 1/10 of the minimum linewidth of the device. For this reason, the feasibility of realizing such an X-ray lithography depends on the development of such an X-ray mask having a high accurate X-ray absorber pattern.
An X-ray mask generally comprises a ring-shaped mask substrate made of silicon or the like, such an X-ray permeable film formed on the mask substrate as a SiC thin film having a very small X-ray absorption factor, and a mask pattern formed on the X-ray permeable film and made of a material having a large X-ray absorption factor.
Such an X-ray mask is usually fabricated by such a method as shown in FIGS. 11(a) to 11(f).
More specifically, as shown in FIG. 11(a), first, a 1 .mu.m-thick SiC film (X-ray permeable film) 2 is deposited on a silicon substrate 1 as a mask substrate by a low pressure chemical vapor deposition (LPCVD) method. The X-ray permeable film is required to be such a self supporting film having a tensile stress which can transmit X-rays therethrough and can be excellent in the permeability to alignment light (visible light). Reported so far as the material of such X-ray permeable film as to satisfy such requirements are BN, Si, SiN and diamond, in addition to SiC.
Thereafter formed on the back side of the silicon substrate 1 of the aforementioned resultant assembly is a Cr film 5 having an opening.
Next, as shown in FIG. 11(b), a 0.5 .mu.m-thick W film 7 is deposited on the SiC film 2. In this case, the X-ray absorbing material is required to have a large X-ray absorption factor at an exposure wavelength and also to be easy in micro-processing. In addition, since the X-ray absorber is present on the X-ray permeable film as thin as 1 .mu.m, it is indispensable that the internal stress of the X-ray absorber is as low as about 1.times.10.sup.7 N/m.sup.2. This is because of the fact that, when the stress of the X-ray absorber is large, this causes the X-ray permeable film to be deformed, which results in that a positional distortion takes place in the X-ray absorber pattern. To avoid this, a sputtering method enabling the stress control is employed to control the internal stress and achieve the desirable deposition.
As shown in FIG. 11(c), a supporting frame 9 made of silicon is then joined by a direct bonding method to the silicon substrate 1.
As shown in FIG. 11(d), pattern writing is carried out with use of an electron beam writing system to form a resist pattern 10.
Then, as shown in FIG. 11(e), the W film 7 is subjected to a patterning process by anisotropic etching with use of the resist pattern 10 as a mask.
Finally, as shown in FIG. 11(f), liquid phase etching with use of a solution of potassium hydroxide (KOH) is carried out over the silicon substrate 1 with the Cr film 5 used as a mask to make an opening having a diameter of 30 mm therein.
Meanwhile, it is necessary for the purpose of raising its X-ray transmissivity that the X-ray permeable film should be formed as very thin as about 1 .mu.m. To this end, in order to minimize the pattern positional distortion caused by the stress of the X-ray absorber pattern, the X-ray permeable film is made of an SiC material having a large Young's modulus and a large Poisson's ratio.
In such a circumstance, a very vital issue to be solved for the practical use of the X-ray mask having the X-ray permeable film made of the SiC material is the level of the visible-light transmissivity of the X-ray permeable film. In other words, since alignment between the X-ray mask and a wafer is achieved with use of a He-Ne laser, it is generally required that the X-ray permeable film has a transmissivity of more than 70% at a wavelength of 633 nm, though the transmissivity varies from X-ray stepper to X-ray stepper. However, the SiC and diamond films have actually transmissivities of about 50-60%. This is considered to result from the fact that the SiC or diamond film has a large refractive index of about 2.3-2.6 and also has a large reflection factor at the interface between air (or He atmosphere) and the X-ray permeable film. For the purpose of overcoming this, there has been suggested such a method that an anti-reflective film is coated on an X-ray permeable film to improve a visible-light tranmissivity (refer to Proceeding of The Fifty-first Meeting of The Japan Society of Applied Physics, p.455).
The requirements of the anti-reflective film are that the refractive index n of the film should be close to the square root of the refractive index of the X-ray permeable film and that the thickness of the film should be an odd-number multiple of .lambda./(4 n)(where .lambda. denotes wavelength). A film having a refractive index of about 1.5-1.6 is suitable as an anti-reflective film and thus an SiO.sub.2 film having a refractive index of 1.45 has been used. Further, for the purpose of minimizing the absorption by the anti-reflective film itself, the thickness of the film is set to be 109 nm (.lambda./4).
However, this proposal has been defective in that, since an X-ray absorber pattern of such heavy metal as W formed on the anti-reflective film is formed by a reactive ion etching process, the anti-reflective film is also subjected to the etching in this etching process. In more detail, during the etching of the X-ray absorber pattern, a micro-loading effect causes a slow etching rate for a fine pattern while causes a fast etching rate for a rough pattern, which means that an etching end point varies with the pattern dimensions. For this reason, when the anti-reflective film as the underlying layer of the X-ray absorber pattern is small in its etching resistance, the anti-reflective film as the underlying layer of the rough X-ray absorber pattern is also subjected to the etching at the end of the etching operation of the fine pattern, which results in that the film thickness is varied and thus a sufficient anti-reflective effect cannot be realized. In actual applications, a gas containing fluorine such as CF.sub.4 or SF.sub.6 is used as an etching gas for the etching of the W film and at the same time, it is used as the etching gas for the SiO.sub.2 film, thus resulting undesirably in a small selection ratio between the both.
For the purpose of avoiding this, there has been proposed such a method that an X-ray absorber pattern is formed and thereafter an anti-reflective film is coated on the X-ray absorber pattern. This method however has a problem that, since the anti-reflective film is also deposited even on the side walls of the X-ray absorber pattern, contrast at the edge portions of the pattern is reduced and thus it becomes impossible to obtain an abrupt resist pattern. In addition, since the anti-reflective film is formed after the formation of the X-ray absorber pattern, its stress control also becomes very difficult.
The above method has another problem that the X-ray permeable film has a surface roughness as large as 50 nm (p-v) while the anti-reflective film formed on the X-ray permeable film is as thin as about 109 nm as already explained above, so that the surface roughness cannot be improved remarkably and this inevitably involves the reduction of its transmissivity caused by surface scattering. For the purpose of making the X-ray permeable film smooth, it is considered to form a thick anti-reflective film, but this imposes a very difficult stress control requirement and also involves the influence of light absorption by the anti-reflective film.
Furthermore, even the formation of the X-ray absorber pattern involves a large etching problem.
More specifically, when it is desired to subject the W film with an X-ray absorber pattern of a 1 G DRAM level and having a minimum linewidth of 0.15 .mu.m to an etching process with use of the resist as an etching mask, for example, its aspect ratio becomes close nearly to 10. In such an etching as to have a large aspect ratio, a micro-loading effect becomes a serious problem. That is, since an etching rate become slow for a fine pattern and becomes fast for a rough pattern, the etching end point varies depending on the pattern dimensions. Further, the sectional shape of the pattern also varies with different pattern dimensions (dimension conversion difference becomes large). For the purpose of reducing the influences of the micro-loading effect, it is necessary to make small the thickness of the etching mask. Because of the small selection ratio between the resist and W film, an SiO.sub.2 film has been employed as a mask material other than the resist in some cases. However, its selection ratio is not sufficient still and the SiO.sub.2 film must be made nearly as thick as the W film. In addition, when the thick etching mask remains on the X-ray absorber, its stress becomes a serious problem.
In this way, in the etching of the X-ray absorber pattern, it is required to make the mask material as thin as possible.
This requirement becomes a big problem not only in the formation of the X-ray absorber pattern but also in the etching of a microfine pattern.
As explained above, in order to improve the visible light transmissivity of the conventional X-ray permeable film, the X-ray permeable film is coated with the anti-reflective film. However, since the anti-reflective film is small in the etching resistance to the etching conditions for the X-ray absorber, it has been impossible to achieve a sufficient anti-reflective effect.
Further, since the surface roughness of the X-ray permeable film is large, even when the anti-reflective film is coated on the X-ray permeable film, this disadvantageously cannot prevent the reduction of the transmissivity caused by the surface scattering and also cannot attain a sufficient anti-reflective effect.
Furthermore, it is highly difficult to form a thin mask material for the etching of a microfine heavy metal pattern and the etching of the microfine pattern requires the realization of a mask pattern having a large etching selection ratio.